All mice contain endogenous mouse mammary tumor virus (MMTV) DNA sequences in their genome. However, different strains of mice differ drastically in their tumor incidence (from less than 1 to 100 percent at 18 months) and in their mode of transmission (milk vs. gametes). The object of this project is to relate differences in expression of mouse mammary tumor virus with differences in the virus specific nucleic acids present in normal and mammary tumor tissue and with the relative arrangement of the MMTV specific DNA sequences in the host cell genome. The proposed work involves the examination of the endogenous BALB/c virus, the investigation of how mouse mammary tumor virus induces tumors in mammary glands of the mouse, and the mapping of the endogenous and exogenous MMTV present in various mouse strains. This work will involve nucleic acid annealing, restriction endonucleases and the Southern blotting procedure, and somatic cell genetics.